1. Field of the Invention
The present invention relates to a steel for the manufacture of high-strength screws and to a high-strength screw made from said steel. More specifically, the present invention relates: to a steel for the manufacture of high-strength screws having a tapping ability for joining a member (in which a prepared hole has been formed) whilst forming a large diameter (M8 or larger) internal thread and having a strength of 800 N/mm2 or more; and to a high-strength screw made from such a steel.
2. Description of the Prior Art
A tapping screw joins members together through forming an internal thread through the members. This can only be achieved if a prepared hole is formed in the members that are to be joined together. In order to be able to use tapping screws to join members together by forming an internal thread the tapping screws must be harder than the members. The tapping screw must be sufficiently harder than the members to be joined in order to cut the thread in the members. This is also important for the joint to be mechanically sound.
For these reasons, a conventional screw, for example, a cross-recessed tapping screw (in accordance with JIS B1122) has been manufactured from carbon-steel wires of SWRCH 12A to 22A (aluminum killed steel) or from SWRCH 12K to 22K (killed steel) (in accordance with JIS G3539) through the processes of forming a screw through rolling the steel, and refining the formed screw by using the techniques of cementation, hardening, and tempering.
One important factor of steel for use in the manufacture of tapping screws is its"" toughness after hardening, therefore aluminum killed steels are used as they have fine crystal grains. However, properties that conflict with toughness (such as hardness and strength) must also reach satisfactory levels as well. Japanese Patent Laid Open No. 9-67625 discloses a tapping screw manufactured from a steel that has a high-magnesium (Mn) content and a low carbon (C) content by a process of cementation, hardening and tempering, that has a surface hardness Hv of 560 to 600 and an internal hardness Hv of 320 to 360. Hereafter, this type of tapping screw is referred to as prior art 1.
Japanese Patent Laid Open No. 10-196627 (1998) discloses a screw manufactured from a low carbon-high Mn steel that has a surface hardness Hv of 550 or higher and an internal hardness Hv of 320 to 400. Hereafter, this type of tapping screw is referred to as prior art 2.
In order to be able to join high strength members, an even higher surface hardness and internal toughness is required in the screw in order to be able to form an internal thread in the members. At present however, the materials and the method for manufacturing such a screw have not been established.
Both prior art 1 and prior art 2 are intended to be used for the manufacture of relatively small diameter screws (for example, smaller than M6). Therefore, if screws or bolts of M8 or larger are manufactured from these materials it is difficult to obtain the well-balanced surface hardness and internal hardness (after cementation) and the required strength.
The object of the present invention is to provide a steel for use in the manufacture of high-strength tapping screws(having a strength of 800 N/mm2 or higher) and for tapping screws or bolts of large diameters (M8 or larger) and also to provide a high-strength screw manufactured from such a steel.
The inventors of the present invention conducted intensive studies in order to solve the above-described problems and obtained the following findings.
The hardness balance of screws and bolts of large diameters after cementation can be controlled and the desired strength can be obtained by:
(1) the addition of a large quantity of Cr,
(2) the adjustment of the ingredients to the adequate DI-value range,
(3) the adequate control of the surface hardness internal hardness and effective depth of the hardened layer, and
(4) the adequate control of the tempering temperature after cementation hardening.
The present invention is based on such findings and is characterized by the following;
The invention is characterized by a steel for high-strength screws comprising(by % mass): C: 0.05 to 0.20, Si: 0.20 or less (not including 0), Mn: 0.5 to 2.0, P: 0.015 or less, S: 0.015 sol. Al: 0.020 to 0.080, N: 0.0060 or less, Cr: more than 0.80 to 2.0 and the balance being iron and unavoidable impurities.
The invention is also characterized by the steel for high-strength screws further comprising (by % mass) of at least one selected from a group consisting of: Ni: 3.5 or less, Cu: 1.0 or less, Mo: 0.30 or less, and B: 0.0005 to 0.0050: and at least one selected from a group consisting of: Ti: 0.005 to 0.050 and Nb: 0.005 to 0.050.
The invention is also characterized by the steel for high-strength screws wherein the DI value represented by the following equation (1) is within a range of between 17 mm and 43 mm.
The invention is also characterized by the steel for high-strength screws wherein the DI value represented by the above equation (1) is within a range between 17 mm and 43 mm.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm2 or more.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm2 or more.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm2 or more.
The invention is also characterized by a high-strength screw wherein the surface hardness Hv after cementation is 550 to 700, the internal hardness Hv after cementation is 200 to 320, the effective depth of the hardened layer is 0.05 to 1.00 mm and the strength of 800 N/mm2 or more.
The invention is also characterized by the high-strength screw wherein tempering is carried out within a temperature range between 200xc2x0 C. and 400xc2x0 C. after cementation.
The invention is also characterized by the high-strength screw wherein tempering is carried out within a temperature range between 200xc2x0 C. and 400xc2x0 C. after cementation.
The reason for limiting the values in the present invention will be described below.
(1) C: 0.05 to 0.20% by mass
xe2x80x9cCxe2x80x9d is an important element in the manufacture of strong steel. If the content of C is less than 0.05% by mass high strength cannot be obtained and cementation-hardenability lowers. If the content of C exceeds 0.20% by mass the internal hardness of the screw becomes too high and the toughness of the steel lowers. Therefore, the content of C was limited to the range between 0.05 and 0.20% by mass.
(2) xe2x80x9cSixe2x80x9d: 0.20% by mass or less (not including 0)
Since Si plays an important role as a deoxidizing agent it is always added to steel in the manufacturing process. It also improves the resistance of the steel to softening (due to tempering and hardenability) and increases the strength of the steel. If the content of Si is too high the resistance to deformation increases and therefore the ability to cold-forge the steel is lowered. The upper limit of the Si content was determined to be 0.20% by mass.
(3) Mn: 0.5 to 2.0% by mass
Similarly to Si, Mn is an element required in the deoxidizing process of steel. It also increases the hardenability of steel. The addition of at least 0.5% Mn by mass is necessary for the steel to reach the required strength. Since Mn (as does P and S) separates on the crystal grain boundary of steel (and therefore increases the brittlement at the grain boundary) the upper limit of the Mn content was determined to be 2.0% by mass.
(4) P: 0.015% by mass or less
P separates on the austenite grain boundary and therefore weakens the boundary and it also dissolves in ferrite to form a solid solution and lowers the deformability of the steel. Since P is an impurity in the present invention the content of P was determined to be 0.015% by mass or less.
(5) S: 0.015% by mass or less
S forms MnS to lower the deformability of the steel and MnS can also become the point from which cracks propagate. Since S is an impurity in the present invention the content of S was determined to be 0.015% by mass or less.
(6) Sol. Al: 0.020 to 0.080% by mass
Al is not only a deoxidizing agent, but also stops N from separating on the grain boundary (fixing it as AlN) and therefore improves the strength of the grain boundary. In order to have this effect on N, the content of Al is 0.020% by mass or higher as sol. Al (acid-soluble Al). However, if the sol. Al content exceeds 0.080% by mass, the aggregate of Al2O3 is formed during the continuous casting of ingots causing the nozzle to be choked and making the casting operation difficult. Therefore, the required content of sol. Al was determined to be within a range of between 0.020 and 0.080% by mass.
(7) N: 0.0060% by mass or less
N causes strain-aging hardening during screw processing to lower the cold-forgeability of steel and also shortens the life of the tools. Since N is an impurity in the present invention the content of N was determined to be 0.0060% by mass or less.
(8) Ti: 0.005 to 0.050% by mass
Ti has the ability to refine crystal grains. If the level of Ti is less than 0.005% by mass the refining effect is small, also the effect to fix N as TiN is also small. However, the addition of Ti in excess of 0.050% by mass not only saturates these effects but also forms large quantities of hard TiN and TiC, lowering forgeability and raising the cost of alloying. The content of Ti was determined to be within a range between 0.005 and 0.050% by mass.
(9) Cr: more than 0.80 to 2.0% by mass
Cr raises the hardenability of steel and also ensures its"" strength. Studies have shown that the addition of Cr in excess of 0.80% by mass is required to ensure the strength of large bolts of M8 or larger. However, since Cr also raises the resistance of the steel to softening due to tempering, the excessive addition of Cr will make the steel too hard, and adversely affects the toughness of the steel. Therefore, the upper limit of the content of Cr was determined to be 2.0% by mass.
(10) Mo: 0.30% by mass or less
Mo is used to prevent the separation of P on a grain boundary, raise the strength of the grain boundary and to improve the hardenability of steel. However, since the excessive addition of Mo inhibits the cold-forgeability of steel (like Cr) and also Mo is an expensive element, the upper limit of Mo was determined to be 0.30% by mass.
(11) B: 0.0005 to 0.0050% by mass
The addition of a trace of B has the ability to improve the hardenability of steel. Also, B forms BN to prevent the separation of N on a grain boundary. The addition of B can lower the amount of Mn, Cr and Mo and further improve the cold-forgeability of steel. In order to make B exert such effects 0.0005% by mass or more B must be added. However, if more than 0.0050% by mass is added boron cementite is precipitated and the grain boundary strength is weakened. The content of B was therefore determined to be within a range of between 0.0005 and 0.0050% by mass.
(12) Nb: 0.005 to 0.050% by mass
Similarly to Ti, Nb has the ability to refine crystal grains. However, since the addition of less than 0.005% by mass of Nb has little effect, the lower limit was determined to be 0.005% by mass. However, (similar to Ti) since Nb has a strong affinity to C and N, it forms carbide or nitride easily and if Nb is added in a large quantity it is deposited on the grain boundary and accelerates brittleness as well as increasing the alloying costs. Therefore, the upper limit of the content of Nb was determined to be 0.050% by mass.
(13) Ni: 3.5% by mass or less
Ni imparts hardenability to steel and raises the static strength of steel. In addition, since Ni improves toughness it is useful to improve the hardenability and the toughness of the steel. However, if it is added excessively the effect becomes saturated and since it is a very expensive element, the upper limit of the content of Ni was determined to be 3.5% by mass.
(14) Cu: 1.0% by mass or less
Cu is also used to improve the hardenability and to raise the static strength of steel. The addition of Cu in an adequate quantity is effective to improve the mechanical properties of steel, although since the addition of too much Cu causes surface defects during hot rolling and causes defective cold forging, the upper limit of Cu was determined to be 1.0% by mass.
(15) Surface Vickers hardness Hv: 550 to 700
This range is required to obtain the required bolt strength and to form an internal thread in the members to be joined. If the Vickers hardness Hv is lower than 550 the tip of the tapping screw cracks or breaks and therefore cannot form the internal thread. If the Hv exceeds 700 the notch effect is raised and the occurrence of cracks will be accelerated. Therefore, the surface hardness Hv of the screw was determined to be within a range between 550 and 700.
(16) Internal Vickers hardness Hv: 200 to 320
Similarly to surface hardness, internal hardness is important to obtain the required bolt strength. If the internal hardness Hv is lower than 200 the required bolt strength is unobtainable. If the Hv exceeds 320 the toughness lowers and cracks can easily occur. Therefore, the internal hardness Hv of the screw was determined to be within a range between 200 and 320.
(17) Tempering temperature: 200 to 400xc2x0 C.
The tempering temperature is directly related to the final performance (surface and internal hardness) of the bolt. If the tempering temperature is lower than 200xc2x0 C. the steel becomes excessively hard, whilst if the tempering temperature exceeds 400xc2x0 C. the steel will not attain the required. Therefore, the tempering temperature was determined to be within a range between 200xc2x0 C. and 400xc2x0 C.
(18) Effective depth of hardened layer: 0.05 to 1.00 mm
In order to be able to form an internal thread in the members that are to be joined a level of hardness is required in the surface of the screw. If the effective depth of the hardened layer is less than 0.05 mm the screws ability to form the internal thread is compromised; whilst if the effective depth exceeds 1.00 mm the internal toughness of the screw is lowered, which will increase the chance of cracks forming. Therefore, the effective depth of the hardened layer was determined to be within a range between 0.05 and 1.00 mm.
(19) DI value (mm): 17 to 43
The DI value (mm) is an index to evaluate the hardenability of steel and is calculated using the following equation (1). If the DI value of the steel is less than 17 mm the steel will not attain the required strength for use as a tapping screw; whilst if the DI value exceeds 43 mm there is a possibility that the toughness of the steel will be reduced. Therefore, the DI value was determined to be within a range between 17 and 43.
DI=25.4xc3x97DIC(*1)xc3x97FSi(*2)xc3x97FMn(*3)xc3x97FCr(*4)xc3x97(FMo(*5), FCu(*6), FNi(*7), FB(*8))xe2x80x83xe2x80x83(1)
where:
*1: DIC=0.54xc3x97(C),
*2: FSi=1.00+0.7xc3x97(Si),
*3: FMn=3.3333xc3x97(Mn)+1.00 (Mnxe2x89xa61.20),
FMn=5.10xc3x97(Mn)xe2x88x921.12 (Mn greater than 1.20),
*4: FCr=1.00+2.16xc3x97(Cr),
*5: FMo=1.00+3.00xc3x97(Mo),
*6: FCu=1.00+0.365xc3x97(Cu),
*7: FNi=1.00+0.363xc3x97(Ni), and
*8: FB=2 (only when B is added).